Synthetic oils were produced as lubricants to overcome the shortcomings in the properties of petroleum oils. In Kirk-Othmer, it is reported, that in 1929, polymerized olefins were the first synthetic oils to be produced commercially in an effort to improve the properties of petroleum oils. The greatest utility of synthetic oils has been for extreme temperatures. Above about 100.degree.-125.degree. C., petroleum oils oxidize rapidly; high viscosity and wax separation generally set a low temperature limit of -20.degree. to -30.degree. C. Outside this range, synthetics are almost a necessity; the same types of additives as those discussed for petroleum oils usually are used. Fire resistance, low viscosity-temperature coefficient, and water solubility are among the unique properties of synthetic oils. Cf. Kirk-Othmer, ENCYCLOPEDIA OF CHEMICAL TECHNOLOGY, "Lubrication and Lubricants, Vol. 14, p496 (1981). As representative synthetic hydrocarbon oils, the Kirk-Othmer reference refers to Mobil 1, SHC 824, and SHC 629 (also products of Mobil Oil Corporation), as well as to silicones, organic esters, phosphates, polyglycols, polyphenyl ethers, silicates and fluorochemicals, Kirk-Othmer, Vol. 14, p497.
The formulation of lubricants typically includes an additive package incorporating a variety of chemicals to improve or protect lubricant properties in application to specific situations, particularly internal combustion engine and machinery applications. The more commonly used additives include oxidation inhibitors, rust inhibitors, antiwear agents, pour point depressants, detergent-dispersants, viscosity index (VI) improvers, foam inhibitors and the like. This aspect of the lubricant arts is specifically described in Kirk-Othmer ENCYCLOPEDIA OF CHEMICAL TECHNOLOGY, 34d edition, Vol. 14, pp477-526, incorporated herein by reference. Considering the diversity of chemical structures represented by the plethora of additives incorporated in a typical lubricant formulation, and the quantity in which they are added, the artisan in the lubricant formulation arts faces a substantial challenge to provide a homogeneous formulation which will remain stable or in solution during inventory and during use. Lubricants, particularly synthetic lubricants of the type of interest in the instant invention, are usually hydrogenated olefins. Due to their hydrocarbon structure they are largely incompatible with polar additives such as antioxidants, antirust and antiwear agents, etc. Accordingly, in order to render the lubricants compatible with the polar additives large amounts of expensive polar organic esters must be added to the formulation. Useful commercial formulations may contain 20 percent or more of such esters as bis-tridecanol adipate for example, solely to provide a fully homogeneous lubricant blend of lubricant and additive.
Modifying the solvent properties of lubricants with solubilizing agents such as organic esters, while solving the problem of how to prepare stable blends with lubricant additives, creates or accentuates other performance related problems beyond the added burden on cost of the product. Accordingly, workers in the field are challenged by the need to incorporate the desirable properties of additives into lubricants, without incurring the usual physical and cost liabilities.
One class of lubricants of particular interest in the present invention are synthetic lubricants obtained by the oligomerization of olefins, particularly C.sub.6 -C.sub.20 alpha olefins. Catalytic oligomerization of olefins has been studied extensively. Many catalysts useful in this area have been described, especially coordination catalyst and Lewis acid catalysts. Known olefin oliogomerization catalysts include the Ziegler-Natta type catalysts and promoted catalysts such as BF3 or AlCl3 catalysts. U.S. Pat. No. 4,613,712 for example, teaches the preparation of isotactic alpha-olefins in the presence of a Ziegler type catalyst. Other coordination catalysts, especially chromium on a silica support, are described by Weiss et al in Jour. Catalysis 88, 424-430 (1984) and in Offen. DE 3,427,319.
Poly alpha-olefin (PAO) oligomers as reported in literature or used in existing lube base stocks are usually produced by Lewis acid catalysis in which double bond isomerization of the starting alpha-olefin occurrs easily. As a result, the olefin oligomers have more short side ranches and internal olefin bonds. These side branches degrade their lubricating properties. Recently, a class of synthetic, oligomeric, polyalpha-olefin lubricants, has been discovered with a regular head-to-tail structure and containing a terminal olefinic bond. These lubricants have shown remarkably high viscosity index (VI) with low pour points and are especially characterized by having a low branch ratio, as defined hereinafter.
Accordingly, it is an object of the present invention to incorporate into that class of lubricants those properties typically associated with lubricant additives.
It is another object of the instant invention to improve properties by incorporating additive functional properties by forming adducts with sulfides or sources of sulfide group incorporation and yet another object is to provide those properties in lubricant compositions free of both zinc and phosphorus.
Yet another object of the instant invention is to improve lubricant properties of mineral oil based and synthetic lubricants by blending with sulfide functional group modified HVI-PAO.